Recently, along with the development of information communication industries, electronic devices become smaller, lighter, slimmer and more portable. In this trend, demands on a high energy density battery used as a power source of such electronic devices have increased. A lithium ion secondary battery is one of the most satisfactory batteries, and numerous studies towards improvements are now in progress actively.
A lithium ion secondary battery includes a cathode, an anode and an electrolyte and a separator that give paths of lithium ions between the cathode and the anode. When lithium ions are intercalated or disintercalated at the cathode and the anode, oxidation and reduction reactions are made to generate electric energy. In an early stage of lithium ion secondary batteries, lithium metal with high energy density was used as an anode, and a liquid solvent was used as an electrolyte. However, this early-stage lithium ion secondary battery exhibits bad life cycle due to dendrite. To solve this disadvantage, there were developed lithium ion secondary batteries in which carbon material capable of absorbing a large amount of lithium ions was used as an anode instead of lithium metal, and an electrolyte was composed of organic liquid or solid polymer.
However, as charging and discharging are repeated for the lithium ion secondary battery using carbon material as an anode, LiF generated by side reaction of LiPF6 used as an electrolyte is irregularly formed on an anode surface, which deteriorates performance of the battery and resultantly shortens the life cycle of the battery.
Korean Patent Registration No. 0433822 and Japanese Laid-open Patent Publication No. 2000-012026 disclose a technique for coating an anode surface with metal or metal oxide, and Korean Patent Registration No. 0324623 discloses a technique for forming a porous metal film on an anode surface. However, all of them are not successful in solving the irregular formation of LiF on an anode surface.